$A=\left[\begin{array}{rr}-15 & 12 & 24 \\ 33 & 8 & 1 \\-9 &-4 &-7\end{array}\right]$ $A_{1,2}=$
Background An $m\times n$ matrix has $m$ rows and $n$ columns. $A=\left[\begin{array}{rr}A_{1,1} & \cdots & A_{1,n} \\\\\vdots \ & \ddots & \vdots \\\\A_{m,1} &\cdots &A_{m,n}\end{array}\right]$ Therefore, the entry $A_{{c},{d}}$ is located on row ${c}$ and column ${d}$. Finding $A_{1,2}$ $A_{{1},{2}}$ is located on row ${1}$ of $A$ : $\left[\begin{array}{rr}{-15} & {12} & {24} \\ 33 & 8 & 1 \\-9 &-4 &-7\end{array}\right]$ $A_{{1},{2}}$ is also located on column ${2}$ of $A$. $\left[\begin{array}{rr}{-15} & {\text{12}} & {24} \\ 33 & 8 & 1 \\-9 &{-4} &-7\end{array}\right]$ Therefore, $A_{{1},{2}}={12}$. Summary $A_{1,2}=12$